15.86 The concentration of nitrate ion can be determined by a multistep process.

Question

15.86 The concentration of nitrate ion can be determined by a multistep process. A 25.00-mL sample of water from a rural well contaminated with NO, (aq) was made basic and treated with an excess of zinc metal, which reduces the nitrate ion to aqueous ammonia. The evolved ammonia gas was passed into 50.00 mL of 2.50 × 10-3 M HCl(aq). The unreacted HCl(aq) was titrated to the stoichiometric point with 28.22 mof .50 x 103 M NaOH(a). (a) Write balanced chemical equations for the three reactions. (b) What is the molar concentration of nitrate ion in the well water?

Explanation / Answer

From the given data

(a) balanced equation,

NO3- to NH3(aq) in basic medium

4Zn + NO3- + 6H2O + 7OH- --> 4Zn(OH)4^2- + NH3

Evolved NH3 titrated with HCl

NH3 + HCl --> NH4Cl

Excess HCl titrated with NaOH

HCl + NaOH --> NaCl + H2O

(b) Total moles HCl used = 2.5 x 10^-3 M x 50 ml = 0.125 mmol

NaOH used = 1.5 x 10^-3 M x 28.22 ml = 0.04233 mmol

moles of HCl reacted with NH3 = 0.125 - 0.04233 = 0.08267 mmol

moles NH3 present = 0.08267 mmol

moles NO3- ion in water = 0.08267 mmol

molarity of NO3- in water sample = 0.08267 mmol/25 ml = 0.00331 M

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